Switching of frequency multiplexed microwave signals using cascading multi-path interferometric josephson switches with nonoverlapping bandwidths

ABSTRACT

A cascading microwave switch (cascade) includes a set of Josephson devices, each Josephson device in the set having a corresponding operating bandwidth of microwave frequencies, wherein different operating bandwidths have different corresponding center frequencies. A series coupling is formed between first Josephson device from the set and an n th  Josephson device from the set, wherein the series coupling causes the first Josephson device in an open state to reflect back to an input port of the first Josephson device a signal of a first frequency from a frequency multiplexed microwave signal (multiplexed signal) and the n th  Josephson device in the open state to reflect back to an input port of the n th  Josephson device a signal of an n th  frequency from the multiplexed signal.

TECHNICAL FIELD

The present invention relates generally to a device, a fabricationmethod, and fabrication system for a frequency multiplexed microwavelight switch usable with superconducting qubits in quantum computing.More particularly, the present invention relates to a device, method,and system for switching of frequency-multiplexed microwave signalsusing cascading multi-path interferometric Josephson switches innonoverlapping bandwidths, where the switches are based on nondegeneratethree-wave-mixing Josephson devices.

BACKGROUND

Hereinafter, a “Q” prefix in a word of phrase is indicative of areference of that word or phrase in a quantum computing context unlessexpressly distinguished where used.

Molecules and subatomic particles follow the laws of quantum mechanics,a branch of physics that explores how the physical world works at themost fundamental levels. At this level, particles behave in strangeways, taking on more than one state at the same time, and interactingwith other particles that are very far away. Quantum computing harnessesthese quantum phenomena to process information.

The computers we use today are known as classical computers (alsoreferred to herein as “conventional” computers or conventional nodes, or“CN”). A conventional computer uses a conventional processor fabricatedusing semiconductor materials and technology, a semiconductor memory,and a magnetic or solid-state storage device, in what is known as a VonNeumann architecture. Particularly, the processors in conventionalcomputers are binary processors, i.e., operating on binary datarepresented in 1 and 0.

A quantum processor (q-processor) uses the odd nature of entangled qubitdevices (compactly referred to herein as “qubit,” plural “qubits”) toperform computational tasks. In the particular realms where quantummechanics operates, particles of matter can exist in multiplestates—such as an “on” state, an “off” state, and both “on” and “off”states simultaneously. Where binary computing using semiconductorprocessors is limited to using just the on and off states (equivalent to1 and 0 in binary code), a quantum processor harnesses these quantumstates of matter to output signals that are usable in data computing.

Conventional computers encode information in bits. Each bit can take thevalue of 1 or 0. These 1s and 0s act as on/off switches that ultimatelydrive computer functions. Quantum computers, on the other hand, arebased on qubits, which operate according to two key principles ofquantum physics: superposition and entanglement. Superposition meansthat each qubit can represent both a 1 and a 0 at the same time.Entanglement means that qubits in a superposition can be correlated witheach other in a non-classical way; that is, the state of one (whether itis a 1 or a 0 or both) can depend on the state of another, and thatthere is more information that can be ascertained about the two qubitswhen they are entangled than when they are treated individually.

Using these two principles, qubits operate as more sophisticatedprocessors of information, enabling quantum computers to function inways that allow them to solve difficult problems that are intractableusing conventional computers. IBM has successfully constructed anddemonstrated the operability of a quantum processor usingsuperconducting qubits (IBM is a registered trademark of InternationalBusiness Machines corporation in the United States and in othercountries.)

A superconducting qubit includes a Josephson junction. A Josephsonjunction is formed by separating two thin-film superconducting metallayers by a non-superconducting material. When the metal in thesuperconducting layers is caused to become superconducting—e.g. byreducing the temperature of the metal to a specified cryogenictemperature—pairs of electrons can tunnel from one superconducting layerthrough the non-superconducting layer to the other superconductinglayer. In a qubit, the Josephson junction—which functions as adispersive nonlinear inductor—is electrically coupled in parallel withone or more capacitive devices forming a nonlinear microwave oscillator.The oscillator has a resonance/transition frequency determined by thevalue of the inductance and the capacitance in the qubit circuit. Anyreference to the term “qubit” is a reference to a superconducting qubitcircuitry that employs a Josephson junction, unless expresslydistinguished where used.

The information processed by qubits is carried or transmitted in theform of microwave signals/photons in the range of microwave frequencies.The microwave signals are captured, processed, and analyzed to decipherthe quantum information encoded therein. A readout circuit is a circuitcoupled with the qubit to capture, read, and measure the quantum stateof the qubit. An output of the readout circuit is information usable bya q-processor to perform computations.

A superconducting qubit has two quantum states —|0> and |1>. These twostates may be two energy states of atoms, for example, the ground (|g>)and first excited state (|e>) of a superconducting artificial atom(superconducting qubit). Other examples include spin-up and spin-down ofthe nuclear or electronic spins, two positions of a crystalline defect,and two states of a quantum dot. Since the system is of a quantumnature, any combination of the two states are allowed and valid.

For quantum computing using qubits to be reliable, quantum circuits,e.g., the qubits themselves, the readout circuitry associated with thequbits, and other parts of the quantum processor, must not alter theenergy states of the qubit, such as by injecting or dissipating energy,in any significant manner or influence the relative phase between the|0> and |1> states of the qubit. This operational constraint on anycircuit that operates with quantum information necessitates specialconsiderations in fabricating semiconductor and superconductingstructures that are used in such circuits.

A microwave switch is a device that allows microwave light waves to passthrough it in a substantially loss-less manner (transmission) when theswitch is in a closed state, and reflects the microwave light waves backto the sender when in an open state (reflection). A reference herein toan “switch” is a reference to a microwave switch. In other words, theswitch operates as a binary microwave light bridge, and the response ofthe device is dependent on the state of the device, regardless of thedirection from which the light signal might be attempting to go acrossthe switch (from port 1 to 2 or from port 2 to 1). Switches are used inquantum computing for allowing or disallowing microwave signals into andout of the quantum processor as needed.

A multi-path interferometric Josephson switch based on nondegeneratethree-wave-mixing Josephson devices is hereinafter compactly andinterchangeably referred to as Multi-Path Interferometric JosephsonSwitch (MPIJSW). An MPIJSW device can be implemented as a microwaveswitch in a superconducting quantum circuit. The MPIJSW is a directionagnostic device whose operation is controlled by a phase of a microwavedrive coupled to the switch.

A superconducting nondegenerate three-wave-mixing device can be used aspart of the MPIJSW by operating the mixing device in a frequencyconversion (no photon gain) mode. The nondegenerate three-wave mixer canbe a Josephson parametric converter (JPC).

A superconducting nondegenerate three-wave mixer has 3 ports, which areSignal port (S) through which a microwave signal of frequency f_(S) canbe input, Idler port (I) through which an idler microwave signal offrequency f_(I) can be input, and pump port (P) through which microwavesignal of frequency f_(P) and phase φ_(p) can be input. In oneconfiguration (without loss of generality), f_(I) is a high frequency,f_(P) is a low frequency, and f_(S) is a medium frequency, when f_(P),f_(S), and f_(I) are compared relative to each other (i.e.,fID>f_(S)>f_(P)). The superconducting nondegenerate three-wave mixer ischaracterized as nondegenerate because it has two modes—namely S and I,which are both spatially and spectrally different.

From Idler to Signal port, the Idler microwave signal enters the Idlerport at frequency f₂, is down converted, and exits the Signal port atfrequency f₁. From Signal to Idler port, the microwave signal enters theSignal port at frequency f₁, is up converted, and exits the Idler portat frequency f₂. The pump microwave signal provides the energy forfrequency up conversion and frequency down conversion. The pumpfrequency is f_(P), where f_(P)=f₁−f_(S)=f₂−f₁.

On resonance, the nondegenerate three-wave mixer (e.g., JPC) satisfiesthe following scattering matrix when operated in noiseless frequencyconversion:

$\lbrack S\rbrack = {\begin{pmatrix}r & t \\t^{\prime} & r\end{pmatrix} = \begin{pmatrix}{\cos \; \theta} & {{ie}^{{- i}\; \phi_{P}}\sin \; \theta} \\{{ie}^{i\; \phi_{P}}\sin \; \theta} & {\cos \; \theta}\end{pmatrix}}$

where tanh(iθ/2)=i|ρ| and ρ is a dimensionless pump amplitude (variesbetween 0 and 1).

As a modification to the nondegenerate three-wave mixer and as furtherrecognized herein, the phase of the pump φ_(p) (which can be denoted asφ₁ and φ₂ for two pump signals) will be utilized in accordanceembodiments described herein. Since the scattering matrix is unitary,the following relation holds |r|²+|t|²=1, where r is the reflectioncoefficient, t is the transmission parameter, and t′=−t* (where t* isthe conjugate of t). Unitary means that the nondegenerate three wavemixer preserves the energy and the coherence of the phase. The frequencyconversion working point of the superconducting nondegeneratethree-wave-mixing device is |r|²=0, |t|²=1. At the frequency conversionworking point, there is no reflection and there is full transmissionwith frequency conversion.

Two suitable manifestations of the nondegenerate three-wave mixer, eachoperating at the same frequency conversion working point are used as onecomponent in an MPIJSW according to the illustrative embodiments. JPC isone such non-limiting manifestation.

In quantum circuits, microwave signals can include more than onefrequency. Generally, the microwave signals span a band of frequencies.An MPIJSW generally operates with a comparatively narrow band offrequencies around a central frequency for which the MPIJSW is tuned.The illustrative embodiments recognize that a new switch design isneeded that is capable of switching signals of all or some microwavesignals having different, even if a frequency of a signal lies outsidethe operational frequency band of a single MPIJSW.

SUMMARY

The illustrative embodiments provide a superconducting device, and amethod and system of fabrication therefor. A superconducting device ofan embodiment forms a cascading microwave switch (cascade), whichincludes a set of Josephson devices, each Josephson device in the sethaving a corresponding operating bandwidth of microwave frequencies,wherein different operating bandwidths have different correspondingcenter frequencies. The embodiment includes a series coupling betweenfirst Josephson device from the set and an n^(th) Josephson device fromthe set, wherein the series coupling causes the first Josephson devicein an open state to reflect back to an input port of the first Josephsondevice a signal of a first frequency from a frequency multiplexedmicrowave signal (multiplexed signal) and the n^(th) Josephson device inthe open state to reflect back to an input port of the n^(th) Josephsondevice a signal of an n^(th) frequency from the multiplexed signal.

Another embodiment further includes an (n−1)^(th) Josephson device fromthe set in the series coupling, wherein n is greater than 1, wherein the(n−1)^(th) Josephson device is included the series coupling between thefirst Josephson device and the n^(th) Josephson device, and wherein the(n−1)^(th) Josephson device in the open state reflects back to an inputport of the (n−1)^(th) Josephson device a signal of an (n−1)^(th)frequency from the multiplexed signal.

In another embodiment, the series coupling causes the first Josephsondevice in the closed state to transmit the signal of the n^(th)frequency from the multiplexed signal through the series coupling andthe n^(th) Josephson device in the open state to transmit the signal ofthe first frequency through the series.

In another embodiment, the series coupling causes the first Josephsondevice when closed and the n^(th) Josephson device when closed topropagate signals of all frequencies from the multiplexed signal in anydirection through the series coupling, wherein the multiplexed signalcomprises a frequency other than the first frequency and the n^(th)frequency.

In another embodiment, a first operating bandwidth of microwavefrequencies corresponding to the first Josephson device isnonoverlapping for at least some frequencies with an n^(th) operatingbandwidth of microwave frequencies corresponding to the n^(th) Josephsondevice.

In another embodiment, a total switching bandwidth of the cascadecomprises the first operating bandwidth and the n^(th) operatingbandwidth.

In another embodiment, the first Josephson device in the set ofJosephson devices is an MPIJSW, which includes a first nondegeneratemicrowave mixer device (first mixer); a second nondegenerate microwavemixer device (second mixer); a first input/output (I/O) port coupled toan input port of the first mixer and an input port of the second mixer;and a second I/O port coupled to the input port of the first mixer andthe input port of the second mixer, wherein the signal of the firstfrequency communicated between the first I/O port and the second I/Oport is transmitted while propagating in either direction between thefirst I/O port to the second I/O port through the first mixer and thesecond mixer when the MPIJSW is closed, and wherein the first frequencyis in a first operating bandwidth of the first Josephson device.

Another embodiment further includes a first microwave pump injecting afirst microwave drive into the first mixer at a pump frequency and afirst pump phase, wherein the first microwave pump is configured tocause the first mixer to operate at a frequency conversion workingpoint; and a second microwave pump injecting a second microwave driveinto the second mixer at the pump frequency and a second pump phasewherein the second microwave pump is configured to cause the secondmixer to operate at the same frequency conversion working point.

In another embodiment, the first mixer and the second mixer are each anondegenerate three-wave mixer.

In another embodiment, the first mixer and the second mixer are each aJosephson parametric converter (JPC), and wherein the first mixer andthe second mixer are nominally identical.

An embodiment includes a fabrication method for fabricating thesuperconducting device.

An embodiment includes a fabrication system for fabricating thesuperconducting device.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofthe illustrative embodiments when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts a block diagram of an example configuration of an MPIJSWthat is usable in a cascade in accordance with an illustrativeembodiment;

FIG. 2 depicts another alternate configuration for an MPIJSW that isusable in a cascade in accordance with an illustrative embodiment;

FIG. 3 depicts a block diagram of an example configuration and a totalreflection operation of a cascading MPIJSW in accordance with anillustrative embodiment;

FIG. 4 depicts a block diagram of an example transmission operation of acascading MPIJSW in accordance with an illustrative embodiment;

FIG. 5 depicts a flowchart of an example process for reflecting ortransmitting signals of all frequencies in a frequency multiplexedmicrowave signal using cascading multi-path interferometric Josephsonswitches with nonoverlapping bandwidths in accordance with anillustrative embodiment;

FIG. 6 depicts a block diagram of an example configuration and aselective switching operation of a cascading MPIJSW in accordance withan illustrative embodiment;

FIG. 7 depicts a flowchart of an example process for propagation orswitching the signals of some but not all frequencies in a frequencymultiplexed microwave signal using cascading multi-path interferometricJosephson switches with nonoverlapping bandwidths in accordance with anillustrative embodiment.

DETAILED DESCRIPTION

The illustrative embodiments used to describe the invention generallyaddress and solve the above-described needs for switching signals ofsome or all frequency-multiplexed microwave signals. The illustrativeembodiments provide a switch device comprising cascading multi-pathinterferometric Josephson switches having nonoverlapping bandwidths,where the switches are based on nondegenerate three-wave-mixingJosephson devices. Such a cascading switch device is compactly referredto herein as a cascading MPIJSW.

An operation described herein as occurring with respect to a frequencyof frequencies should be interpreted as occurring with respect to asignal of that frequency or frequencies. All references to a “signal”are references to a microwave signal unless expressly distinguishedwhere used.

The term “frequency multiplexed signal” refers to a composite signalwhich includes multiple signals at various frequencies and is thereforenot different from the term “frequency multiplexed signals”, whichrefers to signals at various frequencies multiplexed together. The twoterms are therefore used interchangeably to mean more than one signalsof different frequencies multiplexed or presented together to a deviceor in an operation.

An embodiment provides a configuration of a cascading MPIJSW. Anotherembodiment provides a fabrication method for the cascading MPIJSW, suchthat the method can be implemented as a software application. Theapplication implementing a fabrication method embodiment can beconfigured to operate in conjunction with an existing superconductorfabrication system—such as a lithography system.

For the clarity of the description, and without implying any limitationthereto, the illustrative embodiments are described using some exampleconfigurations. From this disclosure, those of ordinary skill in the artwill be able to conceive many alterations, adaptations, andmodifications of a described configuration for achieving a describedpurpose, and the same are contemplated within the scope of theillustrative embodiments.

Furthermore, simplified diagrams of the example mixers, hybrids, andother circuit components are used in the figures and the illustrativeembodiments. In an actual fabrication or circuit, additional structuresor component that are not shown or described herein, or structures orcomponents different from those shown but for the purpose describedherein may be present without departing the scope of the illustrativeembodiments.

Furthermore, the illustrative embodiments are described with respect tospecific actual or hypothetical components only as examples. The stepsdescribed by the various illustrative embodiments can be adapted forfabricating a circuit using a variety of components that can be purposedor repurposed to provide a described function within a cascading MPIJSW,and such adaptations are contemplated within the scope of theillustrative embodiments.

The illustrative embodiments are described with respect to certain typesof materials, electrical properties, steps, numerosity, frequencies,circuits, components, and applications only as examples. Any specificmanifestations of these and other similar artifacts are not intended tobe limiting to the invention. Any suitable manifestation of these andother similar artifacts can be selected within the scope of theillustrative embodiments.

The examples in this disclosure are used only for the clarity of thedescription and are not limiting to the illustrative embodiments. Anyadvantages listed herein are only examples and are not intended to belimiting to the illustrative embodiments. Additional or differentadvantages may be realized by specific illustrative embodiments.Furthermore, a particular illustrative embodiment may have some, all, ornone of the advantages listed above.

With reference to FIG. 1, this figure depicts a block diagram of anexample configuration of an MPIJSW that is usable in a cascade inaccordance with an illustrative embodiment. MPIJSW configuration 100comprises pair 102 of nondegenerate three-wave mixer 102A andnondegenerate three-wave mixer 102B. Each of nondegenerate three-wavemixer 102A and nondegenerate three-wave mixer 102B is operating at thebeam splitter working point (which is one example of a frequencyconversion working point).

Nondegenerate three-wave mixer 102A is configured with physical ports a1(corresponding to signal port S), b1 (corresponding to signal port I),p1 (corresponding to signal port P), and b1′ (corresponding to signalport I). The pump frequency (f_(P)) is a difference between idlerfrequency (f₂) and input signal frequency (f₁) according to expression108.

Nondegenerate three-wave mixer 102B is configured with physical portsa2, b2, p2, and b2′, and pump frequency (f_(P)) in a similar manner.Port b1 of mixer 102A and port b2 of mixer 102B are coupled togetherusing transmission line 103.

Ports 1 and 2 of 90-degree hybrid 104 form ports 1 and 2, respectively,of MPIJSW 100, as described herein. Port a1 of nondegenerate three-wavemixer 102A is coupled with port 3 of hybrid 104. Port a2 ofNondegenerate three-wave mixer 102B is coupled with port 4 of hybrid104.

This configuration 100 of nondegenerate three-wave mixers 102A and 102B,and other possible similarly-purposed configurations using the describedcomponents, is compactly represented as symbol 110. For example, FIG. 2depicts another possible similarly-purposed configuration using thedescribed components. The state of the bar across the contacts insidethe block of symbol 110 represents a transmitting (closed) or reflecting(open) state of the switch for signal from port 1 to port 2 or port 2 toport 1 in symbol 110 (symbol 110 as depicted in this figure shows anopen switch). In other words, MPIJSW 110 transmits the signal from port2 to port 1 (or port 1 to port 2) when closed but reflects an incomingsignal from port 1 back out of port 1 (or reflects an incoming signalfrom port 2 back out of port 2) when open.

This series connection of the MPIJSW devices is not intuitive. In anormal series coupling of electrical or electronic elements, a parameterof the series is limited (e.g., the bandwidth of the series) by theweakest/smallest/lowest value of the parameter in the serial chain. Theentire series of the elements operates at that weakest/smallest/lowestvalue. In contrast, the cascade of MPIJSW devices, due to the specialproperties of the MPIJSW devices used therein, out-of-band signals (asignal frequency that is not in the device's bandwidth) are not actedupon and allowed to simply pass through, and each device acts (switches)only upon that part of the signal that lies in its own bandwidth, thusproviding a non-intuitive additive span in the bandwidth.

With reference to FIG. 2, this figure depicts another alternateconfiguration for an MPIJSW that is usable in a cascade in accordancewith an illustrative embodiment. Hybrid 204 is a 90-degree hybrid and isconfigured with hybrid-less JPC 202A and hybrid-less JPC 202B in amanner substantially as hybrid 104 is configured with nondegeneratethree-wave mixers 102A and 102B in FIG. 1. Configuration 200 uses asingle pump drive in conjunction with hybrid 206 to provide the pumpinput to hybrid-less JPC 202A and hybrid-less JPC 202B. Configuration200 is also represented by symbol 110.

FIGS. 3-5 describe a cascading configuration and a manner of operatingthe same, to transmit or reflect signals of all frequency-multiplexedmicrowave signals having different frequencies. FIGS. 6-7 describe adifferent cascading configuration and a manner of operating the same, toselectively transmit or reflect signals of some but not allfrequency-multiplexed microwave signals.

With reference to FIG. 3, this figure depicts a block diagram of anexample configuration and a total reflection operation of a cascadingMPIJSW in accordance with an illustrative embodiment. This cascadingconfiguration reflects signals of all frequency-multiplexed microwavesignals having different frequencies that are within the bandwidth ofany of the cascaded MPIJSW devices. Each of MPIJSW devices 302 ₁, 302 ₂. . . 302 _(N) is an MPIJSW according to symbol 110. MPIJSW devices 302₁-302 _(N) represent N MPIJSW devices (N>1) that are cascaded inconfiguration 300.

A cascading of MPIJSW devices is a series connection of MPIJSW deviceswhereby one port (port 1 or 2) of the first MPIJSW (302 ₁) is coupled toan external circuit for receiving a microwave signal input; the otherport (port 2 or 1, correspondingly) of the first MPIJSW (302 ₁) iscoupled to one port of the next MPIJSW (302 ₂); the other port of thenext MPIJSW (302 ₂) is coupled to one port of the next MPIJSW, and soon, until a port of N−1^(th) MPIJSW is coupled to a port of the lastMPIJSW (302 _(N)), and the other port of the last MPIJSW (302 _(N)) iscoupled to an external circuit to which cascade 300 provides a microwavesignal output.

Each MPIJSW 302 ₁-302 _(N) is configured in cascade 300 such that eachMPIJSW 302 ₁-302 _(N) when open reflects back an input signal receivedat one of its ports back to the same port (all switches are open).

Furthermore, each MPIJSW 302 ₁-302 _(N) in cascade 300 operates in asubstantially nonoverlapping frequency band. For example, MPIJSW 302 ₁operates in a narrow bandwidth (BW₁) where a center frequency is f₁,i.e., half of BW₁ is below f₁ and including f₁ and half of BW₁ is abovef₁. Therefore, BW₁ is [f₁−BW₁/2 to f₁+BW₁/2]. Similarly, MPIJSW 302 ₂has a center frequency f₂, and BW₂ of [f₂−BW₂/2 to f₂+BW₂/2]. And theMPIJSW devices in the set are defined in a similar manner until MPIJSW302 _(N) has a center frequency f_(N), and BW_(N) of [f_(N)−BW_(N)/2 tof_(N)+BW_(N)/2]. BW₁ . . . BW_(N) do not overlap, or overlap by aninsignificant amount.

An MPIJSW in cascading configuration 300 operates only on signals in thebandwidth of frequencies for which it is tuned. In other words, anMPIJSW will reflect the signals of only those frequencies that fallwithin its operating bandwidth. The MPIJSW will pass in both directionsand regardless of its open or closed state, in a substantially loss-lessmanner, the signals of frequencies outside of the operating bandwidth ofthat MPIJSW.

For example, MPIJSW 302 ₁ will only reflect a signal of a frequency(substantially prevent the signal of the frequency from passing throughMPIJSW 302 ₁) in BW₁ if MPIJSW 302 ₁ is open, but will allow signals offrequencies in BW₂, BW₃, BW₄ . . . BW_(N) to transmit through it toMPIJSW 302 ₂ in a substantially loss-less manner regardless of the stateof MPIJSW 302 ₁ being open or closed. MPIJSW 302 ₁ will allow signals offrequencies in BW₁ to transmit through it to MPIJSW 302 ₂ only whenMPIJSW 302 ₁ is closed. Each MPIJSW 302 ₁ . . . 302 _(N) inconfiguration 300 operates relative to its respective operatingbandwidth and frequencies outside its operating bandwidth in a similarmanner.

In configuration 300, MPIJSW 302 ₁ reflects the signal of frequency f₁because MPIJSW 302 ₁ reflects the signals of frequencies in BW₁ whenopen, and MPIJSW 302 ₁ is open and f₁ is in BW₁. MPIJSW 302 ₁ allowssignals of frequencies f₂ . . . f_(N) to pass regardless of being openbecause those frequencies are outside BW₁. Similarly, MPIJSW 302 ₂reflects the signal of frequency f₂ because MPIJSW 302 ₂ reflects thesignals of frequencies in BW₂ when open, and MPIJSW 302 ₂ is open and f₂is in BW₂. The signal of frequency f₁ never reached MPIJSW 302 ₂ due tothe reflection from MPIJSW 302 ₁. MPIJSW 302 ₂ allows signals offrequencies f₃, f_(i) . . . f_(N) to pass because those frequencies areoutside BW₂. MPIJSW 302 _(N) reflects the signal of frequency f_(N)because MPIJSW 302 _(N) reflects the signals of frequencies in BW_(N)when open, and MPIJSW 302 _(N) is open and f_(N) is in BW_(N). Thesignals of frequencies f₁ . . . f_(N-1) never reached MPIJSW 302 _(N)due to the reflection from MPIJSW 302 ₁ . . . 302 _(N-1). Thus, asdepicted in this figure, an input signal that multiplexes frequenciesf₁, f₂ . . . f_(N) is completely reflected by cascade 300.

Configuration 300 is represented compactly as cascading MPIJSW 302. Theeffective bandwidth over which cascading MPIJSW 302 can reflect istherefore,

BW={[f ₁−BW₁/2 to f ₁+BW₁/2], [f ₂−BW₂/2 to f ₂+BW₂/2], . . . [f_(N)−BW_(N)/2 to f _(N)+BW_(N)/2]}

The reflection bandwidth BW of cascading MPIJSW 302 is greater than thereflection bandwidth of any single MPIJSW in configuration 300. Thus,cascading MPIJSW 302 is operable over a broader bandwidth than theoperating bandwidth of a single MPIJSW.

With reference to FIG. 4, this figure depicts a block diagram of anexample transmission operation of a cascading MPIJSW in accordance withan illustrative embodiment. Cascade 300, MPIJSW devices 302 ₁, 302 ₂ . .. 302 _(N), and cascading MPIJSW 302 are the same as in FIG. 3.Frequency f₁ is in BW₁ of MPIJSW 302 ₁, f₂ is in BW₂ of MPIJSW 302 ₂ . .. f_(N) is in BW_(N) of MPIJSW 302 _(N).

In the transmission operation of this figure, all switches are closed.Signals of frequencies f₁, f₂ . . . f_(N) are input at an input port ofcascade 300, e.g., at port 1 (or port 2) of the first MPIJSW (302 ₁) incascade 300. In cascade 300, MPIJSW 302 ₁ transmits the signal offrequency f₁ to the next switch (MPIJSW 302 ₂) because MPIJSW 302 ₁transmits the signals of frequencies in BW₁ and f₁ is in BW₁. MPIJSW 302₁ also transmits signals of frequencies f₂ . . . f_(N) to the nextswitch because they are out of bandwidth BW₁. Operating in this manner,MPIJSW 302 ₁ has effectively transmitted signals of all frequencies fromthe multiplexed input microwave signal. Similarly, each of 2 throughN^(th) MPIJSW transmits signals of frequency f₁ . . . f_(N) by similarreasoning pertaining to their respective bandwidths BW₂ . . . BW_(N).

Operating in this manner, each MPIJSW when closed transmits a signal ofthat frequency which is within the bandwidth of that MPIJSW andtransmits signals of those frequencies which are outside the bandwidthof that MPIJSW. Thus, cascade 300 effectively performs completetransmission of the frequency multiplexed microwave signal input.

Cascading MPIJSW 302 according to configuration 300 has the effectivebandwidth over which cascading MPIJSW 302 can transmit as,

BW={[f ₁−BW₁/2 to f ₁+BW₁/2], [f ₂−BW₂/2 to f ₂+BW₂/2], . . . [f_(N)−BW_(N)/2 to f _(N)+BW_(N)/2]}

Again, the transmission bandwidth BW of cascading MPIJSW 302 is greaterthan the bandwidth of any single MPIJSW in configuration 300. Thus,cascading MPIJSW 302 is operable to transmit a frequency multiplexedsignal that spans a broader bandwidth than the operating bandwidth of asingle MPIJSW.

With reference to FIG. 5, this figure depicts a flowchart of an exampleprocess for reflecting or transmitting signals of all frequencies in afrequency multiplexed microwave signal using cascading multi-pathinterferometric Josephson switches with nonoverlapping bandwidths inaccordance with an illustrative embodiment. Process 500 can beimplemented using cascading MPIJSW 302 for the operations described inFIGS. 3 and 4.

Each Josephson device in a set of Josephson devices is configured as anMPIJSW (block 502). The MPIJSW devices are connected in a cascade byconnecting one MPIJSW with another MPIJSW in a series connection (block504). The MPIJSW devices in the series connection are configured suchthat all MPIJSW devices in the series reflect or transmit a microwavesignal of their respective frequency at the same time in the cascade.The cascade is built by adding all MPIJSW devices from the set in seriesin this manner (block 506).

The cascade operates to reflect (as in FIG. 3) or transmit (as in FIG.4) an input microwave signal containing a signal at a frequencycorresponding to the bandwidth of any of the MPIJSW devices in theseries (block 508).

FIGS. 6-7 now describe a different cascading configuration and a mannerof operating the same, to selectively reflect (and therefore alsoselectively transmit) signals of some but not all frequencies of afrequency multiplexed microwave signal.

With reference to FIG. 6, this figure depicts a block diagram of anexample configuration and a selective switching operation of a cascadingMPIJSW in accordance with an illustrative embodiment. This cascadingconfiguration reflects signals of only some frequencies in a frequencymultiplexed microwave signal. Each of MPIJSW devices 602 ₁, 602 ₂ . . .602 _(N) is an MPIJSW according to symbol 110. MPIJSW devices 602 ₁-602_(N) represent N MPIJSW devices (N>1) that are cascaded in configuration600.

A cascading of MPIJSW devices is a series connection of MPIJSW deviceswhereby an MPIJSW in the series can be connected such that one or moreMPIJSW devices are open and one or more MPIJSW devices are closed at thesame time. For example, non-limiting example cascade 600 is formed bycoupling a port of the first MPIJSW (602 ₁, which is closed in exampleconfiguration 600) to an external circuit for receiving a frequencymultiplexed microwave signal input. The other port of the first MPIJSW(602 ₁) is coupled to a port of the next MPIJSW (602 ₂, which is open inexample configuration 600). The other port of MPIJSW 602 ₂ is coupled toa port of the next MPIJSW, and so on, until a port of N− 1^(th) MPIJSWis coupled to a port of the last MPIJSW (602 _(N), which is closed inexample configuration 600). The other port of the last MPIJSW (602 _(N))is coupled to an external circuit to which cascade 600 provides amicrowave signal output.

Without implying any limitation, and only for the clarity of thedescription, example configuration 600 is depicted with only one MPIJSW(602 ₂) open. Any number of MPIJSW devices can be coupled in series andopened, and any number of MPIJSW devices can be coupled in series andclosed, to construct a cascade that selectively reflects signals ofcertain frequencies. The cascade constructed in this manner reflectsback signals of those frequencies which correspond to the MPIJSW devicesthat are open, and transmits signals of those frequencies whichcorrespond to those MPIJSW devices that are closed.

Thus, depending upon which group of signal frequencies from a frequencymultiplexed microwave signal have to be transmitted, one or more MPIJSW602 ₁-602 _(N) having bands corresponding to those frequencies areconfigured in cascade 600 as closed. And depending upon which signalfrequencies from a frequency multiplexed microwave signal have to bereflected, one or more MPIJSW 602 ₁-602 _(N) having bands correspondingto those frequencies are configured in cascade 600 as open.

Furthermore, each MPIJSW 602 ₁-602 _(N) in cascade 600 operates in asubstantially nonoverlapping frequency band. For example, MPIJSW 602 ₁operates in a relatively narrow bandwidth (BW₁) where a center frequencyis f₁, i.e., half of BW₁ is below f₁ and includes f₁ and half of BW₁ isabove f₁. Therefore, BW₁ is [f₁−BW₁/2 to f₁+BW₁/2]. Similarly, MPIJSW602 ₂ has a center frequency f₂, and BW₂ of [f₂−BW₂/2 to f₂+BW₂/2]. Andthe MPIJSW devices in the set are defined in a similar manner until the(N−1)^(th) MPIJSW has a center frequency f_(N-1), and BW_(N-1) of[f_(N-1)−BW_(N-1)/2 to f_(N-1)+BW_(N-1)/2]; and MPIJSW 602 _(N) has acenter frequency f_(N), and BW_(N) of [f_(N)−BW_(N)/2 tof_(N)+BW_(N)/2]. BW₁ . . . BW_(N) do not overlap, or overlap only by aninsignificant amount.

An MPIJSW in cascading configuration 600 reflects only the signals ofthat bandwidth of frequencies for which it is tuned. In other words, anMPIJSW when open will reflect (flowing in any direction from port 2-1 orport 1-2 of that MPIJSW) signals of those frequencies that fall withinits operating bandwidth. The MPIJSW will pass in both directions, in asubstantially loss-less manner, signals of frequencies outside of theoperating bandwidth of that MPIJSW regardless of that MPIJSW being openor closed.

For example, MPIJSW 602 ₂ will only reflect a signal of a frequency inBW₂ if MPIJSW 602 ₂ is open, but will allow signals of frequencies inBW₁, BW₃, BW₄ . . . BW_(N-1), BW_(N) to pass in a substantiallyloss-less manner regardless of the state of MPIJSW 602 ₂. When closed,MPIJSW 602 ₂ will transmit signals of frequencies not only in BW₂ butalso in BW₁, BW₃, BW₄ . . . BW_(N) in a substantially loss-less mannerin any direction (port 1-2 or port 2-1). Each MPIJSW 602 ₁ . . . 602_(N) in configuration 600 operates relative to its respective operatingbandwidth and frequencies outside its operating bandwidth in a similarmanner.

In configuration 600, MPIJSW 602 ₁ transmits a signal of frequency f₁ tothe next MPIJSW (MPIJSW 602 ₂) because MPIJSW 602 ₁ transmits thesignals of frequencies in BW₁ when closed, MPIJSW 602 ₁ is closed, andf₁ is in BW₁. MPIJSW 602 ₁ transmits signals of frequencies f₂ . . .f_(N) because those frequencies are outside BW₁. However, MPIJSW 602 ₂is configured in cascade 600 in an open state, and therefore reflectsthe signal of frequency f₂ because MPIJSW 602 ₂ reflects the signals offrequencies in BW₂ when open, MPIJSW 602 ₂ is open, and f₂ is in BW₂.MPIJSW 602 ₂ transmits signals of frequencies f₁, f_(i) . . . f_(N-1),f_(N) because those frequencies are outside BW₂. In the reflection path,f₂ being out-of-band BW₁, MPIJSW 602 ₁ allows signal at f₂ to betransmitted in the reverse direction so that the signal at f₂ isreflected back to the sender on cascade 600's input port. Assuming thatall other MPIJSW devices in cascade 600 are configured as closed, themultiplexed signal with signals at f₁, f₃ . . . f_(i) . . . f_(N-1),f_(N) (no f₂) reaches MPIJSW 602 _(N). MPIJSW 602 _(N) transmits thesignal of frequency f_(N) because MPIJSW 602 _(N) transmits the signalsof frequencies in BW_(N) when closed, MPIJSW 602 _(N) is closed, andf_(N) is in BW_(N). MPIJSW 602 _(N) transmits signals of frequencies f₁,f₃ . . . f_(i) . . . f_(N-1) because those frequencies are outsideBW_(N). Thus, as depicted in this figure, an input signal thatmultiplexes frequencies f₁, f₂ . . . f_(N) is transmitted by cascade 600in a substantially loss-less manner (zero or negligible attenuation) inselected frequencies f₁, f₃ . . . f_(i) . . . f_(N) with the signal offrequency f₂ having been selectively reflected back from the inputsignal.

To generalize, if input signal (at one port of the cascade) has signalsof frequencies f_(A), f_(B), f_(C), f_(D), f_(E), f_(F), f_(G), andf_(H), MPIJSW A (reflects signal at f_(A)), C (reflects signal atf_(C)), E (reflects signal at f_(E)), G (reflects signal at f_(G)) areclosed, and MPIJSW B (reflects signal at f_(B)), D (reflects vf_(D)), F(reflects signal at f_(F)), and H (reflects signal at f_(H)) are open,then the output signal (at the other port of the cascade) will containonly signals of f_(A), f_(C), f_(E), and f_(G) and signals of f_(B),f_(D), f_(F), and f_(H) will be reflected.

The effective bandwidth over which cascade 600 can selectively reflect(and therefore selectively transmit) signals of certain frequencies istherefore,

BW={[f ₁−BW₁/2 to f ₁+BW₁/2], [f ₂−BW₂/2 to f ₂+BW₂/2], . . . [f_(N)−BW_(N)/2 to f _(N)+BW_(N)/2]}

The reflecting or transmitting bandwidth BW of cascade 600 is greaterthan the reflecting or transmitting bandwidth of any single MPIJSW inconfiguration 600. Thus, cascade 600 is operable with a frequencymultiplexed signal that spans a broader bandwidth than the operatingbandwidth of a single MPIJSW.

With reference to FIG. 7, this figure depicts a flowchart of an exampleprocess for propagation or switching the signals of some but not allfrequencies in a frequency multiplexed microwave signal using cascadingmulti-path interferometric Josephson switches with nonoverlappingbandwidths in accordance with an illustrative embodiment. Process 700can be implemented using cascade 600 for the operations described inFIG. 6.

Each Josephson device in a set of Josephson devices is configured as anMPIJSW (block 702). The MPIJSW devices are connected in a cascade byconnecting one MPIJSW with another MPIJSW in a series connection (block704). The MPIJSW devices in the series connection are configured suchthat at least some MPIJSW devices (open MPIJSW devices) in the seriesreflect a microwave signal of their respective frequency back to theinput port of the cascade. The cascade is built by adding all MPIJSWdevices from the set in series in this manner (block 706).

The cascade operates to selectively reflect (as in FIG. 6) a frequencymultiplexed input microwave signal where the signal contains a frequencycorresponding to any of the open MPIJSW devices in the series (block708).

The circuit elements of the MPIJSW device and connections thereto can bemade of superconducting material. The respective resonators andtransmission/feed/pump lines can be made of superconducting materials.The hybrid couplers can be made of superconducting materials. Examplesof superconducting materials (at low temperatures, such as about 10-100millikelvin (mK), or about 4 K) include Niobium, Aluminum, Tantalum,etc. For example, the Josephson junctions are made of superconductingmaterial, and their tunnel junctions can be made of a thin tunnelbarrier, such as an aluminum oxide. The capacitors can be made ofsuperconducting material separated by low-loss dielectric material. Thetransmission lines (i.e., wires) connecting the various elements can bemade of a superconducting material.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layer “C”) isbetween layer “A” and layer “B” as long as the relevant characteristicsand functionalities of layer “A” and layer “B” are not substantiallychanged by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “illustrative” is used herein to mean “serving asan example, instance or illustration.” Any embodiment or designdescribed herein as “illustrative” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. The terms“at least one” and “one or more” are understood to include any integernumber greater than or equal to one, i.e. one, two, three, four, etc.The terms “a plurality” are understood to include any integer numbergreater than or equal to two, i.e. two, three, four, five, etc. The term“connection” can include an indirect “connection” and a direct“connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A cascading microwave switch (cascade)comprising: a set of Josephson devices, each Josephson device in the sethaving a corresponding operating bandwidth of microwave frequencies; anda series coupling between the Josephson devices from the set, whereinthe series coupling reflects from a frequency multiplexed microwavesignal (multiplexed signal) a signal of a frequency corresponding to anopen Josephson device in the series coupling.
 2. The cascade of claim 1,wherein the Josephson device in the open state reflects the signal ofthe frequency back to an input port of the Josephson device, and whereinthe frequency is in an operating bandwidth of the open Josephson device.3. The cascade of claim 1, wherein the series coupling causes a firstJosephson device in a closed state to transmit a signal of an n^(th)frequency from the multiplexed signal through the series coupling and ann^(th) Josephson device in an open state to transmit a signal of a firstfrequency through the series, wherein the first frequency corresponds tothe first Josephson device and the n^(th) frequency corresponds to then^(th) Josephson device.
 4. The cascade of claim 1, wherein the seriescoupling causes the Josephson device when closed to propagate signals ofall frequencies from the multiplexed signal in any direction through theseries coupling, wherein the multiplexed signal comprises a frequencyother than the frequency.
 5. The cascade of claim 1, wherein a firstoperating bandwidth of microwave frequencies corresponding to theJosephson device is nonoverlapping for at least some frequencies with ann^(th) operating bandwidth of microwave frequencies corresponding to ann^(th) Josephson device.
 6. The cascade of claim 5, wherein a totalswitching bandwidth of the cascade comprises the first operatingbandwidth and the n^(th) operating bandwidth.
 7. The cascade of claim 1,wherein the Josephson device in the set of Josephson devices is anMPIJSW, comprises: a first nondegenerate microwave mixer device (firstmixer); a second nondegenerate microwave mixer device (second mixer); afirst input/output (I/O) port coupled to an input port of the firstmixer and an input port of the second mixer; and a second I/O portcoupled to the input port of the first mixer and the input port of thesecond mixer, wherein the signal of the first frequency communicatedbetween the first I/O port and the second I/O port is transmitted whilepropagating in either direction between the first I/O port to the secondI/O port through the first mixer and the second mixer when the MPIJSW isclosed, and wherein the frequency is in a first operating bandwidth ofthe Josephson device.
 8. The cascade of claim 7, further comprising: afirst microwave pump injecting a first microwave drive into the firstmixer at a pump frequency and a first pump phase, wherein the firstmicrowave pump is configured to cause the first mixer to operate at afrequency conversion working point; and a second microwave pumpinjecting a second microwave drive into the second mixer at the pumpfrequency and a second pump phase wherein the second microwave pump isconfigured to cause the second mixer to operate at the frequencyconversion working point.
 9. The cascade of claim 7, wherein the firstmixer and the second mixer are each a nondegenerate three-wave mixer.10. The cascade of claim 7, wherein the first mixer and the second mixerare each a Josephson parametric converter (JPC), and wherein the firstmixer and the second mixer are nominally identical.
 11. A method to forma cascading microwave switch (cascade), the method comprising:fabricating a set of Josephson devices, each Josephson device in the sethaving a corresponding operating bandwidth of microwave frequencies; andforming a series coupling between the Josephson devices from the set,wherein the series coupling reflects from a frequency multiplexedmicrowave signal (multiplexed signal) a signal of a frequencycorresponding to an open Josephson device in the series coupling.
 12. Asuperconductor fabrication system which when operated to fabricate acascading microwave switch (cascade) performing operations comprising:fabricating a set of Josephson devices, each Josephson device in the sethaving a corresponding operating bandwidth of microwave frequencies; andforming a series coupling between the Josephson devices from the set,wherein the series coupling reflects from a frequency multiplexedmicrowave signal (multiplexed signal) a signal of a frequencycorresponding to an open Josephson device in the series coupling. 13.The superconductor fabrication system of claim 12, wherein the Josephsondevice in the open state reflects the signal of the frequency back to aninput port of the Josephson device, and wherein the frequency is in anoperating bandwidth of the open Josephson device.
 14. The superconductorfabrication system of claim 12, wherein the series coupling causes afirst Josephson device in a closed state to transmit a signal of ann^(th) frequency from the multiplexed signal through the series couplingand an n^(th) Josephson device in an open state to transmit a signal ofa first frequency through the series, wherein the first frequencycorresponds to the first Josephson device and the n^(th) frequencycorresponds to the n^(th) Josephson device.
 15. The superconductorfabrication system of claim 12, wherein the series coupling causes theJosephson device when closed to propagate signals of all frequenciesfrom the multiplexed signal in any direction through the seriescoupling, wherein the multiplexed signal comprises a frequency otherthan the frequency.
 16. The superconductor fabrication system of claim12, wherein a first operating bandwidth of microwave frequenciescorresponding to the Josephson device is nonoverlapping for at leastsome frequencies with an n^(th) operating bandwidth of microwavefrequencies corresponding to an n^(th) Josephson device.
 17. Thesuperconductor fabrication system of claim 16, wherein a total switchingbandwidth of the cascade comprises the first operating bandwidth and then^(th) operating bandwidth.
 18. The superconductor fabrication system ofclaim 12, wherein the Josephson device in the set of Josephson devicesis an MPIJSW, comprises: a first nondegenerate microwave mixer device(first mixer); a second nondegenerate microwave mixer device (secondmixer); a first input/output (I/O) port coupled to an input port of thefirst mixer and an input port of the second mixer; and a second I/O portcoupled to the input port of the first mixer and the input port of thesecond mixer, wherein the signal of the first frequency communicatedbetween the first I/O port and the second I/O port is transmitted whilepropagating in either direction between the first I/O port to the secondI/O port through the first mixer and the second mixer when the MPIJSW isclosed, and wherein the frequency is in a operating bandwidth of thefirst Josephson device.
 19. The superconductor fabrication system ofclaim 18, further comprising: a first microwave pump injecting a firstmicrowave drive into the first mixer at a pump frequency and a firstpump phase, wherein the first microwave pump is configured to cause thefirst mixer to operate at a frequency conversion working point; and asecond microwave pump injecting a second microwave drive into the secondmixer at the pump frequency and a second pump phase wherein the secondmicrowave pump is configured to cause the second mixer to operate at thefrequency conversion working point.
 20. The superconductor fabricationsystem of claim 18, wherein the first mixer and the second mixer areeach a nondegenerate three-wave mixer.